Development of the aircraft and ship building, car making and a number of other industries require new materials having improved workability and service properties.
Metallic structural materials (alloys) are nowadays produced by melting the base metal to liquid form with additive components, with the melting process going at the temperature of the entire system which ensures the complete melting and mutual dissolution of the components (FIG. 2a).
With the drop of temperature of the alloy during cooling and solidification, the solubility of the alloy components sharply decreases and, at a certain temperature particular for each alloy system and composition, solid phases begin to precipitate and grow from the homogeneous melt in the form of alloy component crystals, or, more frequently, in the form of the crystals of the chemical compounds of components (intermetallic phases) (FIG. 2, b,c). With further cooling the rest of the melt is crystallized in the form of a solid solution of the components in the base metal (FIG. 2, d). Intermetallic phases with crystal lattice and properties different from those of the base alloy (matrix) strongly affect the properties of the alloy system as a whole.
The size of the intermetallic phases precipitated in the process of crystallization of the alloy should not exceed fractions of one micron, otherwise quality of the alloy will be sharply impaired due to loss of ductility and strength.
The solubility of metals and metalloids in the metallic matrix is very much limited in the solid state and this factor accounts for the narrow selection of commercial alloys and the practically achieved limit of improvement in the properties of the commercial structural alloys by change in composition.
A new class of structural materials have been developed, which contain artificially incorporated particles or fibers of oxides, carbides and other compounds enabling the attainment of assured properties of the system as a whole. Such materials are known as composites since the components of the metallic system are not precipitated from the matrix metal, as is the case with the conventional alloys, but are artificially incorporated into the system. All known metallic alloys representing the matrix with incorporated particles, whose properties significantly differ from the matrix, are basically the composites, although of natural occurrence in the making of the alloy.
The properties of metallic materials represented by a composite system of artificial or natural origin are indicated as follows:
ductility of the material is determined by ability of the matrix (as a rule the ability of the solid solutions of components in the base alloy) for plastic flow, as well as by size and syngonia (crystalline structure) of intermetalloid and other inclusions in the matrix); PA1 strength, heat resistance, fatigue strength, resistance of materials to development of cracks is determined by interaction of the of the inclusions and the matrix, as well as distortions of the crystalline lattice of the matrix under action of inclusions; PA1 hardness, wear resistance, tribotechnical properties of the material are determined by properties of the inclusions; PA1 modulus of elasticity, linear expansion factor, specific weight (density) of the material are determined by a set of properties of the matrix and inclusions. PA1 produotion of the basic melt; PA1 uniform distribution of solid particles in a mass molten metal; PA1 crystallization of the resultant composite material. PA1 mechanical stirring of the melt and added particles; PA1 pressing pellets mixed powered matrix metals and reinforcing particles followed by plunging the particles to the melt and mechanical stirring of the melt; PA1 dispersion of particles in melt by ultrasound irradiation. PA1 application of metal-philic coatings on the surface of the reinforcing filler particles; PA1 introduction of surfactants into the base metal melt; PA1 increase of the melt temperature.
Thus, the development of new metallic materials with a predetermined combination of workability and service properties should be theoretically achievable on the basis of selection of the optimum composition of the metallic system in each case, that is selection of the matrix and inclusions whose properties and interaction determine the properties of the composite system as a whole.
Selection of the metallic system base (matrix) is determined by required service properties of the material and level of its properties (steel, aluminum, copper, magnesium, nickel, etc.).
The major difficulty in implementation of the technology for production of structural metallic materials is the injection of components into the structure in the form of superfine particles of compounds thermodynamically and thermally stable in the matrix, and which measure from a few nanometres to a few microns.
In the production of natural composite metallic materials (i.e. complex alloys) this problem is dealt with by precipitation of particles (intermetalloids) from supersaturated solid solutions of the components of the alloy in the base metal produced by the use of high-rate cooling of homogeneous melts The required cooling rate can be practically achieved only in case of relatively small quantities of alloy melt In practice, a high cooling rate is provided by physical dispersion of the melt followed by cooling fine drops of the melt in a cooling medium This requires expensive operations of drying, degassing and compacting particles (granules) to provide pellets. Thus, the technology for production of new metallic alloys by the pelletizing technique has not found wide use in the industry.
The difficulty of introducing superfine particles into the metallic melts in attributed to two circumstances. First due to lack of fluidity of superfine particles (thousandths of microns or less in size) the metering of particles when injected into the melt is rather difficult or sometimes even impossible. Second, due to presence of adsorbed oxygen on the surface of the particles upon in contact with the melt, oxides of the base metal are formed on the surface, which prohibits wetting of the particles by the melt. This problem especially manifests itself during injection of the particles into the melts of metals having high oxygen reactivity (aluminum, magnesium, etc.). The above factor also inhibits implementation of such techniques as the direct modification of the alloys by injection of particles--crystallization nuclei into the melt, alloying the melts by injection of alloy components in the form of the powder, use of powdered waste of alloying materials (e.g. silicon) in production of alloys, in particular those of aluminum-silicon system.
One of the most important features of the proposed technology and devices for its implementation is the possibility of injection into the melt of fine particles of the filler materials (in case of production of composites) or structural components (in case of production of alloys), with the formation of the alloy structure following the scheme shown in FIG. 2A.
The matrix free from the atoms of the component is injected with particles of a desired filler material (FIG. 3a). When equilibrium of the system exists between the structural component (Ax By) and solution of the alloy component B in the matrix A, particles incorporated into the matrix dissolve to the concentration of saturation at the appropriate temperature with the decrease in size, this process is highly controllable and enables production of alloys with structure with alloy a predetermined component of limited solubility.
Major stages of a process for the production of cast composite materials involved are described in "Solidification, Structures and Properties of Cast Metal-Ceramic Particle Composites"--Rohatgi P. K., Asthana R., Das S.--Inst. Metal Rev.,--1986--Vol. 31, N3--pp. 15-139 and include:
The following methods have been used in the prior art for injection of superfine particles into a melt as described in "Cast Aluminum-Graphite Particle Composites--a Potential Engineering Material"--Rohatgi P. K., Das S., Dan T. K.--J. Inst. Eng.,--March, 1989--Vol. 67, N2--pp. 77-83:
Problems encountered in the production of cast metal composites relate to lack of or low wetability of the reinforcing filler particles with the matrix melt, as well as non-uniformity of the cast material due to large differences in densities between the matrix and the filler material.
Increase in the strength of the bond between the reinforcing filler particles and the base metal matrix is achieved by a number of techniques as described in "Wetability of Graphite to Liquid Aluminum and the Effect of alloying Elements on It", Choh Takao, Kemmel Roland, Oki Takeo--Z. Metallklunde"--1987--Vol. 78, N4--pp. 286-290, i.e.:
There is also known a method for production of composites (Application No. 56-141960, Japan, dated Aug. 4, 1980 (No. 55-45955), published May 11, 1981) in which is suggested the use as a filler of natural hollow microspheres 150 micron in diameter sufficiently compatible with various metallic materials, as well as graphite powders, TiB.sub.2, aluminum nitride and oxide, flaky and chipped graphite and calcium metal is added to the melt in quantity of 0.05-5.0 wt. % to ensure uniformity of materials.
The major disadvantage of this method is the necessity for introduction into the melt of an element (calcium) which is soluble in the liquid base metal, but practically insoluble in the case solid matrix and which forms a brittle eutectic component with the matrix. This results in lowered mechanical properties of the matrix and of the composite itself. Besides, the use, as a filler, of hollow microspheres of the recited sizes (150 micron) does not help to improve absolute values of mechanical properties and can result only in some improvement in their relative values per unit of mass.
Prior art relevant to the present invention is the method for production of composite materials (Met. Trans., 1978, v. 9 N 3, pp. 383-388) using the base molten metals--Mg. Al, Fe, Ni, Cr, Co doped with insoluble oxide particles (Al.sub.2 O.sub.3, BeO, CaO, CeO.sub.2, TiO.sub.2, MgO, ThO.sub.2, VO.sub.2, ZrO.sub.2), carbides, borides, nitrides of Nb, Ta, Hf, Ti, Zr sized 0.01-10 micron. The particles are injected as powder or thin fibers To ensure uniform distribution of the particles in the melt they are injected in a stream of preheated inert gas (Ar, He) while vigorously stirring the base metal. Volume percentage of particles may range from 0.5 to 20%. Also one of the elements which improve the surface activity at the interface the particle-melt is injected into the molten metal. Injection of such surface active metals (Mg, Si, Ti, Zr, V, Nb) ensures formation of a metalphilic casing on the oxides which significantly improves wetability in the system and there is no segregation in the melt over a period of 30 min.
The foregoing method has the following disadvantages:
1) the chemical composition of the matrix melt is limited by need to inject surface active metals which in a number of cases may lead to impairment of technological and mechanical properties of the resulting composite material;
2) the absence of stirring in the course of solidification promotes, especially in case of a long solidification time, the formation of segregated and laminated areas, and consequently quality of the resulting composite material is lowered;
3) insolubility of the reinforcing particles excludes the possibility of using this method for production of materials with the matrix reinforced with superfine particles of those elements or their compounds which are traditional strengtheners in production of materials by joint crystallization of the base metal with alloying additives and subsequent thermo-mechanical working.